Free cutting alloys



Patented Apr. 13, 1937 UNITED STATES PATENT OFFICE FREE cn'r'rnvc ALLOYSPennsylvania No Application December 28, 1935,

Serial No. 56.547

' Claims. (01. 15-142) The invention relates to aluminum base alloys andis particularly concerned with aluminum base alloys containing copperand magnesium.

Aluminum base alloys containing between 3 per 5 cent and 12 per cent ofcopper have been widely used heretofore. The copper imparts good castingcharacteristics and increases the tensile strength, yield strength, andhardness of the alloy. In the lower portion of the copper range thealloy may be mechanically deformed bythe well known commercial processessuch as rolling, forging, drawing, or extrusion. Alloys containing morethan about 6 per cent copper are generally used in the cast condition.Throughout the en- 15 the range of 3 to 12 per cent copper, however,

the alloys are susceptible to variations and improvements in theirphysical properties by thermal treatments.

The addition of magnesium to the foregoing aluminum-copper alloys inamounts of from 0.1

to 1 per cent causes the alloy to age spontaneously at room temperatureafter a solution heat treatment. Alloys of thisnature develop a muchhigher strength and hardness by heat treatment 5 than it is possible toobtain-by cold working alone.

An even higher strength can be obtained by reheating the alloy to arelatively low temperature after solution heat treatment. These alloysare particularly adapted to use in structures that are highly stressedin service. Because of the high strength which is attainable in thesealloys when heat treated and aged, they are generally used in thiscondition. Our invention which is described hereinbelow, is directed toimproving the machining quality of such alloys.

There are, however, some applications wherein aluminum-copper-magnesiumalloys as hereinabove disclosed might be conveniently and profitablyused except for an inherent disadvantage which militates against theiruse in the production of certain articles requiring exacting machiningoperations. Mechanical cutting operations such as drilling, shaping, orlathe-cutting are successfully carried out only by using certain.

precautions which increase the cost of production and perhaps favor thechoice of another metal or alloy which can be machined more readily butwhich is not so desirable in other respects, as for example, in physicalproperties.

When alloys are difl'lcult to machine this disadvantage becomes evident,in means, through rapid wear of the cutting tool edge, so that frequenttool re-sharpening is required. Despite continual lubrication themachined surface is rough and irregular, and the chip has a tendency to'form a continuous curl or spiral which often fouls the tool or themoving parts of the machine. It is immediately apparent that there is.need for an alloy of good working characteristics and satisfactoryphysical properties, yet possessing such favorable machining propertiesthat the complete machining operations may be performed economically andsuccessfullm'and may be productive of a pleasing surface appearance.

Accordingly an object of our invention is the provision of an aluminumbase alloy containing from. about 3 per cent to about 12 per cent ofcopper 'and from about 0.1 to about 1 per cent of magnesium which may bereadily and economically machined.

Our invention resides in the discovery that the foregoing object iseflected by the addition of two or more of the elements lead, tin,thallium, cadmium or bismuth. The aluminum-coppermagnesium alloys towhich these elements are added in the proportions specified below, areknown as free cutting or free machining alloys because they can bemachined more rapidly than similar alloys without these elements and yethave as good ora better finished surface. After an extended series ofinvestigations we have discovered that these five metals when added toaluminum-copper-magnesium alloys, form aclass of alloying elements byreason of their favorable eflect upon the machining properties of thesealloys. In recognition of this eifect we term lead, tin, thallium,cadmium and bismuth free machining elements. We have further discoveredthat the simultaneous presence of two or more of these elements isproductive of an improvement in free machining characteristics 7 whichis considerably greater than that caused by the presence of the sametotal amount of a single free machining element. For example,theaddition of 0.5 percent of lead and 0.5 percent of bismuth to analuminum base alloy containing about 5 per cent of copper, 0.5 per centmagnesium, effects a greater improvement in machining quality than doesthe addition of 1.0 per cent of either lead or bismuth singly.

These five elements, we believe, are unique with respect to their efiecton the machining charac teristics of aluminuin-copper-magnesium alloys.It is a fortunate circumstance, therefore, that they are also ofrelatively low melting point, a fact which makes possible their additionto molten aluminum in the pure state, without the intervention ofso-called rich alloys". As a matter of fact, we have observed that ofall the metals whose melting point is lower than about 327 C., themelting point of lead, the flve elements we have selected are the onlyones which are commercially suitable and that impart free cuttingcharacteristics but do not have undesirable effect on the fundamentalphysical properties of the base alloy. 7

The total amount of free machining elements should not be less thanabout 0.05 per cent since below this amount there is scarcely anyadvantageous effect. We have determined that a maximum limit of about 6per cent total of two or more of the free machining elements issufficient for satisfactory commercial results, since although the freemachining effect persists beyond thisamount, certain of the otherphysical properties may be unfavorably affected.

Aluminum-copper-magnesium alloys containing two or more of the freemachining elements, lead, tin, thallium, cadmium and bismuth may bemachined more rapidly, with less tool wear, less tool sharpening, betterquality of chip and better machined surface than the same base alloyswithout the free machining additions, and in fact better than the samebase alloys containing an equivalent total amount of a single freemachining element.

Since aluminum base alloys'containing from 3 to 12 per cent of copperhave a wide variety of applications we list several alloys each of whichmay be said to be preferred for a particular purpose. As an alloy formechanical deformation we suggest an aluminum base alloy containing 5per cent of copper, 0.5 per cent of magnesium, and a total of 1 per centof free cutting constituents, for example, 0.5 per cent of bismuth and0.5 per cent of cadmium, the balance being aluminum. For an alloy withexcellent casting characteristics to be used in the unworked conditionwe suggest an alloy containing 10 per cent ofcopper, 0.5 per cent ofmagnesium and a total of 3v per cent of free machining elements, thebalance being aluminum.

For certain purposes, notably the improvement of tensile strength,hardness and grain structure, the alloys as hereinabove disclosed may beimproved by the addition of molybdenum, vanadium, titanium, tungsten,zirconium and chromium. From 0.05 to 1 per cent of any one of theseelements may be used alone, but if more than one is employed the totalamount should not exceed about 2 per cent.

It is characteristic of the five elements lead.

' tin, thallium, cadmium and bismuth that they form with-aluminumaseries of alloys of limited liquid solubility. We have reason tobelieve that the free machining elements are the only ele-- ments whichexhibit this characteristic, with the possible e'i'ic'eption of severalmetals which-are not regarded as .having any commercial promise asadditions to aluminum base.alloys. Within the range disclosed andclaimed however the free machining elements may be added without unusualdifliculty. We suspect that this characteristic feature of the disclosedelements may be one of the significant factors which contribute to theirfree machining effect. We believe that this effect is furtherstrengthened by distributing scribed hereinabove may be subjected to thethermal treatments well known in the art to improve their strength andhardness. We, have found that a solution heat treatment and subsequentaging does not impair the free machining quality of the alloys and inmany instances the treatment even tends to improve this property. Formany purposes a relatively high strength and hardness are necessary tothe successful performance of the machined article and hence the alloymust be heat treated. This treatment is generally applied prior to themachining operation.

As hereinabove indicated the free machining elements, by reason of theirlow melting point, may be added to the molten aluminum alloy in puremetallic form. However, since some difficulty may be encountered inintroducing them in the higher percentages of our disclosed range weprefer to use the method which is more fully described in U. S. PatentNo. 1,959,029, issued March 15, 1934. Briefly it involves heating themelt to a somewhat higher temperature than is customary, and vigorouslystirring it in excess of a critical period of time.

The term aluminum as used herein and in the appended claims embraces theusual impurities found in aluminum ingot of commercial grade or pickedup in the course of the ordinary handling operations incident to meltingpractice.

We claim:

1. An aluminum base alloy consisting of about 5 per cent copper, "0.5per cent magnesium, 0.5 per cent lead and 0.5 per cent bismuth, thebalance being aluminum.

2.v .An aluminum base alloy consisting of about 5 per cent copper, 0.5per cent magnesium, 0.5 per cent bismuth and 0.5 per cent cadmium, thebalance being aluminum.

- 3. A free cutting alloy containing from 3 to 12 per cent of copper,from 0.1 to 1 per cent of magnesium, and a total of} from 0.05 to 6 percent of at least two of the elements from the following metals, lead,tim -thallium, cadmium, and bismuth, to improve its' machiningproperties, the balance being substantially aluminum.

4. A free cutting alloy containing from 3 to 12 per cent of copper, from0.1 tov 1 per cent of magnesium, from 0.05 to 2 per cent of hardeningmetal from the group composed of molybdenum, vanadium, titanium,tungsten, zirconium and chromium, and a total of from 0.05 to 6 per centleast two of the elements from the following metals, lead, tin,thallium, cadmium, and bismuth, to improve its machining properties, thebalance being substantially aluminum. I

5. An aluminum base alloy consisting of about 5 per cent copper, 0.5 percent magnesium, 0.5 per cent cadmium, and 0.5 per cent lead, the balancebeing aluminum.

LOUIS W. KEMPF. WALTER A. DEAN.

CERTIFICATE OF CORRECTION.

?atent 1N0. 2,076,573. April 13,1937.

LOUIS W. KEMPF, ET AL,

It is hereby certified that error appears in the printed specificationof the above numbered p'atent requiring correction as follows: Page 2,second column, line 56, claim 4, after "per cent" vinsert the words ofat; and that the said Letters Patent 'should be readwith this correctiontherein that the same may conform to the recorder the case in the PatentOffice.

Signed and sealed this 1st day of June, A. 'D. 1937.

.r Henry Van Arsdale [Seal) Acting Commissioner of Patents.

